Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in the field.
Conventional internal combustion engines have at least one cylinder, a piston in the cylinder, and a crankshaft driven by the piston. Most of these engines operate on a four stroke cycle of the piston per two revolutions of the crankshaft. During the cycle, the piston's strokes are first outward for intake, first inward for compression, second outward (after ignition) for combustion and power, and second inward for exhaust. The burnt gas is driven out during the exhaust stroke and a fresh charge is drawn in during the intake stroke. These two strokes require little force and the piston is subject to low pressures. These two strokes also require one entire revolution of the crankshaft for these purposes.
More output could be obtained from a four stroke engine of a given displacement if it could complete its cycle in only one revolution of the crankshaft. There are conventional two-stroke engines in which the four functions of combustion, exhaust, intake and compression, are crammed into two strokes of the piston per one revolution of the crankshaft. Such two-stroke engines generally weigh less than four-stroke engines but are generally less fuel efficient than four-stroke engines, and hence are conventionally used only in certain special fields, such as small garden engines.
There is a way to combine the advantages of four strokes of the piston with the advantage of one revolution of the crankshaft per cycle and that is to split the piston into an inner part which closes one end of the combustion chamber and a separable outer part which is connected to the crankshaft, and to provide means to move the inner piston part independently of the outer piston part during exhaust and intake. This provides for the inner piston part to operate on the four-stroke principle during a single revolution of the crankshaft.
U.S. Pat. No. 857,410 discloses that a quarter revolution of meshed gearing can be used to operate the piston parts in their different cycles. This design has many problems such as gnashing of teeth when the two gears engage on each revolution of the drive shaft, and a complicated gearing system that is fixed at a four to one ratio that divides the four strokes in equal lengths and periods.
U.S. Pat. No. 1,413,541 discloses a split piston having a four stroke inner piston part and a two stroke outer piston part (per cycle or engine revolution). There is also provided an inner piston part that has a cycle with a period for each stroke that is exactly 90 degrees and equal to half the period of a stroke of the outer piston which is 180 degrees. Another limitation of the apparatus includes equal stroke lengths or piston travel for the four strokes of the inner piston part.
U.S. Pat. Nos. 857,410 and 1,413,541 each disclose drive connections for the part of the piston that closes the combustion chamber so that it must move in four equal strokes, each completed during a quarter turn (90 degrees).
U.S. Pat. No. 1,582,890 discloses two pistons in a cylinder, which close two chambers. Operating not on a four stroke principle, it uses a cam actuation means to move the inner piston between the two chambers and two sets of ports generally located at opposite ends of its stroke along the cylinder wall. This is to allow the inner piston to pressurize the outer chamber on its downward stroke, which takes a lot of power and strength requiring its actuating apparatus to be unnecessarily heavy and bulky in structure. Furthermore, the outer ports on the cylinder wall limit the inner piston to equal stroke lengths and symmetrical periods. This patent teaches cylinder ports which the inner piston must cover during combustion and final compression of the combined charges from both cylinder chambers, so that these two strokes are limited to equal lengths and shaft turns.
U.S. Pat. No. 5,243,938, incorporated by reference herein, discloses a differential stroke piston apparatus for reciprocating internal combustion engines having a piston means disposed within a cylinder including an inner piston part which closes and seals the cylinder chamber and an outer piston part which serves as a carrier for the inner piston part and is connected to the engine shaft, preferably a crankshaft. The inner piston part is effective to operate on a cycle different from that of the outer piston, for example four strokes for the inner piston part and two strokes for the outer piston part per revolution of the engine. The present invention also provides a differential stroke cycle means to vary the stroke period and/or stroke length of the inner piston part cycle. The preferred embodiment provides a differential-four-stroke inner piston part and an outer piston part that is connected by a connecting rod to a crankshaft during the whole cycle. The two piston parts combine to ride on the connecting rod during the power and compression portions of the cycle, when compression forces are at their highest levels. During the exhaust and intake portions of the cycle, when compression forces are much lower, the inner piston part executes an inward and outward movements that are exhaust and intake respectively, independently of the outer piston part which continues to move connected to the connecting rod.
There is a need in the art for further accommodating (or softening, or cushioning) seating of the inner piston part and/or to adjust the piston seat height on the outer piston part.